Highly swellable absorption medium with reduced caking tendency

ABSTRACT

The present invention concerns highly swellable absorption mediums with a reduced caking tendency at high humidity and/or high temperatures, wherein a swellable polymer is coated with a non-ionic, nitrogen-containing surfactant and optionally a Lewis acid and then reacted by heating.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to highly swellable absorptionmediums having a reduced caking tendency in a moist environment and/orat high temperatures. The present invention also relates to theproduction and application of this absorption medium in hygiene articlesand in technical fields.

[0002] Polymers that absorb aqueous fluids, termed superabsorbers, areknown from a large number of publications. They are modified naturalpolymers and partially or totally synthetic polymers. Totally syntheticpolymers are usually produced by radical polymerization of differenthydrophilic monomers in aqueous solution using a variety of methods. Ingeneral, cross-linking agents are polymerized as well so that thepolymer is no longer water-soluble but only water swellable. Polymersbased, for example, on (meth)acrylic acids can be used as superabsorbersand are partially in the neutralized form as the alkali salt.

[0003] Superabsorbent polymers are usually used in the form ofgranulates as absorbing components in many hygiene articles such asdiapers, feminine pads or absorbent dressings. Producing such articlesrequires exact proportions to be used that can only be guaranteed byconstant conveying in the production plant. The highly hygroscopicnature of superabsorbent polymers causes problems at constant conveyingspeeds. This hygroscopic nature results in caking of the polymerparticles, in particular when the humidity is relatively high and/orwhen the temperature is high. Agglomerated superabsorbers cannot bedosed precisely and stick to the walls of the production plant,resulting in increased cleaning costs. Thus, there have been manyattempts in the past to develop superabsorbent polymers with a reducedcaking tendency.

[0004] Many known processes reduce the hygroscopic nature by addingfinely divided inorganic powder to the surface of the polymer particles.European patent EP 0 388 120 A describes the surface treatment ofpolymers with silicon dioxide powder with an average particle size of0.1 to 30 μm.

[0005] United States patent U.S. Pat. No. 4,734,478 discloses polymersto which, after polymerization, a mixture of a polyalcohol and ahydrophilic organic solvent is added followed by heat treatment at >90°C. Subsequently, the surface cross-linked polymers are treated with0.01% to 10% by weight of silica dust with a particle diameter of lessthan 10 μm. Such polymers are stated to have a high water uptakecapacity as well as a reduced caking tendency.

[0006] U.S. Pat. No. 4,286,082 discloses processes for the production ofwater absorbing resins, in which at least one water-soluble, surfaceactive reagent is added to the monomer solution and the polymer obtainedis heat treated at a temperature of 100° C. to 230° C. The surfaceactive reagents are preferably nonionic surfactants with an HLB of 7 to20. To reduce the caking tendency of such polymers, the polymers aremixed with ultramicroscopic silica.

[0007] Since the dust content of such polymers is increased by treatingwith an inorganic powder, problems with dust arise, in particular whenunder mechanical stress such as the friction resulting from pneumaticconveying. Such a release of dust is preferably avoided for healthreasons, so such polymers are more difficult to manipulate duringproduction and use.

[0008] Attempts to produce polymers with a low dust content aredescribed in U.S. Pat. No. 5,994,440. Such polymers are obtained bycoating the surface of water-absorbent, cross-linked polymers withhydrophilic organic compounds that do not penetrate into the internalstructure of the polymer. Suitable organic compounds are aliphaticpolyols with a molecular weight of more than 200 g/mol. The surfacecoating causes the polymer dust to adhere to the polymer particles or tothe wall of the storage container so that dust can be avoided. The loosedust portion of such a polymer is stated to be ≦2.5 ppm, with dustparticles with a diameter of ≦10 μm being counted.

[0009] In another series of known processes, the surface of theabsorbent particles is treated with hydrophobic agents to reduce thehygroscopic nature. Thus, EP 0 755 964 A2 describes highly swellablehydrogels the surface of which is coated with wax. Any wax with noreactive groups that can react with the carboxyl groups of the polymersurface can be used. Preferably, waxes with a melting point range of 30°C. to 180° C. are used.

[0010] EP 0 509 708 A1 discloses polymers obtained by surfacecross-linking with polyhydroxy compounds and by coating the surface withsurfactants with an HLB between 3 and 10. The polyhydroxy compounds canbe any compound that has at least two hydroxyl groups and that can reactwith the carboxyl groups on the polymer particles. Preferred polyhydroxycompounds include polyglycols or lower glycol derivatives. Particularsurfactants that can be used are sorbitan fatty acid esters, ethoxylatedsorbitan fatty acid esters, glycerin or polyglycerin fatty acid estersor modified surface active polyesters.

[0011] A disadvantage of that process for surface coating polymers withhydrophobic substances is that the hydrophilic nature of the polymersurface is reduced, resulting in reduced liquid uptake rates.

[0012] U.S. Pat. No. 5,728,742 A discloses a non caking, non dustycomposition obtained by treating water absorbing, lightly cross-linkedpolymers with an anti-caking agent and a hydrophilic de-dusting agent.Such de-dusting agents are either polyols with a molecular weight ofmore than 200 g/mol or polyalkylene glycols with a molecular weight of400 to 6000 g/mol. The anti-caking agents are cationic surfactants, forexample quaternary ammonium or phosphonium salts.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The aim of the present invention is to provide superabsorbentpolymers that have a reduced caking tendency compared with absorptionmediums that are known from the prior art, in particular in a moistenvironment such as high humidity and/or high temperatures, with atleast equivalent properties, in particular at least an unchanged wateruptake capacity, retention and uptake rate for water or aqueous fluids,in particular body fluids.

[0014] The present invention provides a highly swellable absorptionmedium with a reduced caking tendency in a moist environment and/or athigh temperatures based on the following components:

[0015] I a water- or aqueous fluid-absorbing natural polymer modifiedwith acid groups or a water-insoluble, optionally surface cross-linked,water- or aqueous fluid-absorbing cross-linked polymer based onpolymerized monomers containing at least partially neutralized acidgroups, which is treated with:

[0016] II at least one coating agent selected from the group formed bynitrogen-containing, non-ionic surfactants; and whereby the mixtureformed from components I and II has been heat treated.

[0017] Preferably, the surfactant in the absorption medium of theinvention is at least one compound with general formula I:

[0018] in which

[0019] R₁ is a z-substituted aliphatic residue, preferably az-substituted, saturated or unsaturated, linear or branched aliphatic C₁to C₃₀ hydrocarbon residue, more preferably C₈ to C₂₂, which optionallycarries aryl residues, preferably a phenyl residue, a z-substitutedbenzene residue, optionally condensed with five or six-membered ringsoptionally containing heteroatoms such as oxygen, phosphorus ornitrogen;

[0020] R₂ is

[0021] a hydrogen; or

[0022] an aliphatic residue, preferably a saturated or unsaturated,linear or branched C₂ to C₂₄ hydrocarbon residue, preferably C₈ to C₂₂;

[0023] a hydroxyalkylene residue,

[0024] the hydroxyl group of said hydroxyalkylen residue is preferablyan end group and/or optionally alkoxylated with 1 to 50, preferably 1 to20, more particularly with 1 to 10 alkylene oxide units, preferablyethylene and/or propylene oxide units, and/or said hydroxyl group isoptionally esterified with a carbon acid, preferably a C₁- to C₈-carbonacid, and

[0025] the alkylene group of said hydroxyalkylen residue is a C₁- toC₈-, preferably C₁- to C₄-hydrocarbon group, occurs in the alkyleneresidue, or a N,N-dihydroxyalkylene-amino-alkylene residue with C₁-C₄ ineach alkylene residue;

[0026] R₃, which may be identical or different, has the same meaning asR₂, provided that with amide compounds, at least one of residues R₂ orR₃ represents a hydroxyalkylene residue or an alkoxylatedhydroxyalkylene residue or a corresponding esterified or alkoxylated oresterified and alkoxylated hydroxyalkylene residue with the definitiongiven for R₂ above;

[0027] n is 0 or 1, preferably 1;

[0028] and z is a whole number from 1 to 4.

[0029] Preferably, component I is a powder. The particle size of thispowder is preferably at least 20% by weight, preferably at least 50% byweight and more preferably at least 70% by weight in the range 150 to1000 μm. Preferably again, less than 20% by weight, more preferably lessthan 10% by weight of the particles of the powder is less than 150 μm.The proportions given by weight in this paragraph all refer to thepowder as a whole. The particle size can be determined usingERT-420.1-99.

[0030] It has surprisingly been discovered that coating water or aqueousfluid-absorbing polymers with at least one coating agent II of theinvention possibly in combination with a Lewis acid III can produceabsorption mediums with a significantly reduced caking tendency, whilethe other technical properties, in particular retention and absorptionunder load, are not affected. Further, the treated polymers have areduced dust production.

[0031] The absorption medium of the invention exhibits at least one,preferably all of the following properties:

[0032] (a) no anti-caking after at least 3, preferably at least 6 andparticularly preferably at least 24 h, most preferably in the range 3 to30 hours, heat treatment being in accordance with the anti-caking testsdescribed below;

[0033] (b) a retention of at least 20 g/g, preferably at least 25 g/gand particularly preferably at least 30 g/g, most preferably in therange 20 to 100 g/g;

[0034] (c) an absorption under load at a load of 0.9 psi (AUL_(0.9 psi))of at least 7 g/g, preferably at least 15 g/g and particular preferablyat least 20 g/g, most preferably in the range 7 to 40 g/g.

[0035] Combinations of two or more of the above properties each producepreferred embodiments of the absorption medium of the invention;combinations ab, ac, bc are preferred, with combinations ab and ac beingparticularly preferred.

[0036] Partially synthetic or totally synthetic polymers can beconsidered in addition to natural polymers as the water swellablehydrophilic polymers. Natural polymers modified with acidic groups,preferably carboxyl groups, that can be used are polysaccharides withcarboxyl groups, preferably starches, celluloses, guar, for examplecarboxymethyl guar, xanthan gum, alginates, gum arabic,carboxymethylcellulose, carboxymethyl starches and mixtures of thesepolysaccharides. These polymers are water swellable and partially ortotally water-insoluble.

[0037] Partially synthetic and totally synthetic polymers are preferablyused, in particular anionic (meth)acrylic acid based polymers, which arein the partially neutralized form as alkali salts, in particular sodiumand/or potassium salts. The degree of neutralisation of the acidicmonomer components can vary, but is preferably in the range 25 mole % to85 mole %. Homo- and co-polymers can be used that are obtained solelyfrom acrylic acids and/or methacrylic acids, from such monomers togetherwith one or more other monomers or alone from one or more othermonomers, but, for example, they can also be grafted anionic polymers,for example based on (meth)acrylic acids, in the partially neutralizedform as the alkali salt, for example graft polymers with polyvinylalcohol, polysaccharides such as starches or cellulose or derivativesthereof or with polyalkylene oxides such as polyethylene oxides orpolypropylene oxides.

[0038] Examples of monomers that can be used to produce the polymers inaddition to (meth)acrylic acids are methyl, ethyl, and(poly)hydroxyalkylesters of (meth)acrylic acids, (meth)acrylamide,crotonic acid, maleic and fumaric acids, itaconic acid,2-acrylamido-2-methylpropanesulphonic acid, vinylsulphonic acid andvinylphosphonic acids and the methyl, ethyl and poly(hydroxyalkyl)estersand amides of these acids, amino- and ammonium group-containing estersand amides of all said acids and water-soluble N-vinylamides, but alsoall other monomers usually employed as elemental units in the productionof superabsorbent polymers can be contained in the polymer. The polymersare preferably cross-linked. Examples of suitable cross-linkingcompounds that can be used to produce the absorbent polymers and containtwo or more reactive groups are polyglycidyl compounds such aspolyglycidyl ether, methylene bis(meth)acrylamide, bis-acrylamido aceticacid, esters of unsaturated acids with polyols or alkoxylated polyols,for example ethylene glycol di(meth)acrylate or trimethylolpropanetri(meth)acrylate or allyl compounds, such as allyl(meth)acrylate,polyallyl esters, tetra-allyloxyethane, triallylamine,tetra-allylethylenediamine or allylesters of phosphoric acid as well asvinyl phosphonic acid derivatives or mixtures thereof. The proportion ofcross-linking agents added during production of the absorbent polymer ispreferably 0.01% to 20% by weight, more preferably 0.1% to 3% by weightwith respect to the total monomer quantity.

[0039] Polymer production is carried out using known methods such asthat described in German patent DE-C1-40 20 780 and which is herebyincorporated by reference and constitutes part of the disclosure.Preferably, the polymer is produced by polymerization in an aqueoussolution using the gel polymerization process.

[0040] The polymer powder obtained by disintegrating, drying andgrinding of the polymer gel can then undergo surface cross-linking.

[0041] Prior to surface cross-linking, the polymer is preferably dried,ground and screened to obtain the desired grain size fraction, then thesurface cross-linking reaction is carried out. In some cases, however,it is pertinent to add the surface cross-linking agent before drying thepolymer gel or before disintegrating the partially or essentially drypolymer. A surface cross-linking step that can be carried out inaccordance with the invention has been described in U.S. Pat. No.4,666,983 and DE-C-40 20 780. These documents are hereby incorporated byreference and thus constitute part of the disclosure. Preferably, thesurface cross-linking agent is often added in the form of a solution inwater, an organic solvent or a mixture thereof, in particular when smallamounts of surface cross-linking agent are used. Examples of suitablemixing machines for adding the surface cross-linking agents are aPatterson-Kelley mixer, a DRAIS turbulence mixer, a Lödige mixer, aRuberg mixer, a worm mixer, a pan mixer and a fluidised bed mixer, alsocontinuous vertical mixers in which the powder is mixed at a highfrequency using rotating knives (Schugi mixer). After the surfacecross-linking agent has been mixed with the polymer, it is heated totemperatures of 60° C. to 250° C., preferably 135° C. to 200° C. andparticularly preferably 150° C. to 185° C. to carry out the surfacecross-linking reaction. The heating period must be limited so that theproperties of the polymer are not affected by the heat treatment.

[0042] Preferred post cross-linking agents for surface cross-linking ofthe polymers are organic cross-linking agents, i.e., compounds thatreact with the surface COOH groups of the polymers, such as alkylenecarbonates, for example 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one,4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one,4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one,1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one,4,6-dimethyl-1,3-dioxan-2-one or 1,3-dioxepan-1-one. Particularlypreferred compounds are 1,3-dioxolan-2-one und4-methyl-1,3-dioxolan-2-one.

[0043] Further, the following compounds can be used as surfacecross-linking agents: polyhydroxy compounds, for example ethyleneglycol,propyleneglycol, diethyleneglycol, dipropyleneglycol, triethyleneglycol,tetraethyleneglycol, tetrapropyleneglycol, polyethyleneglycol,polypropyleneglycol, 1,3-propanediol, glycerine, polyglycerine,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, trimethylolpropane,pentaerythritol or sorbitol; amino alcohols such as diethanolamine,triethanolamine. Further organic surface cross-linking agents that arenot preferred on the grounds of their toxicity and so have to beseverely limited in their use are: polyepoxides such as ethyleneglycoldiglycidylether, polyethyleneglycol diglycidylether,glycerolpolyglycidylether polyglycerol polyglycidylether,propyleneglycol diglycidylether, polypropyleneglycol diglycidylether,glycidol; polyisocyanates such as 2,4-toluenediisocyanate andhexamethylenediisocyanate; halogenated epoxides such as epichlor- andepibromhydrin and α-methyl-epichlorhydrin; polyamine compounds such asethylenediamine, diethylenetriamine, triethylenetetramine,polyallylamine or polyethyleneimine. Additional surface cross-linkingagents that can be used are polyoxazolin compounds such as1,2-ethylenebisoxazolin. The organic surface cross-linking agent ispreferably used in amount of 0.01% to 5% by weight, more preferably 0.1%to 2.5% by weight and particularly preferably from 0.1% to 1% by weight,with respect to the polymer.

[0044] Non-ionic, nitrogen-containing surfactants can be used as coatingagent II, preferably compounds with general formula I. However, amixture of at least two compounds with this formula can also be used.Preferably, coating agent II is based on a fatty acid such as caprylicacid, caprinic acid, lauric acid, myristic acid, palmitic acid,palmitoleic acid, magaric acid, stearic acid, oleic acid, linoleic acid,linolenic acid, arachic acid, or eurecasic acid.

[0045] In particular, coating agent II is a fatty acid alkanolamide, thecorresponding ethoxylated and/or propoxylated compounds, the esterifiedcompounds; or fatty acid amines, the corresponding ethoxylated and/orpropoxylated compounds, which can also be esterified. These includelauric acid monoethanolamide, coconut acid monoethanolamide, stearicacid monoethanolamide, ricinic acid monoethanolamide, undecylenic acidmonoethanolamide, lauric acid diethanolamide, coconut aciddiethanolamide, soya acid diethanolamide, linoleic acid diethanolamide,laurylmyristinic acid diethanolamide, oleic acid diethanolamide, lauricacid isopropanolamide, coconut acid isopropanolamide, oleic acidisopropanolamide, undecylenic acid polydiethanolamide, coconut acidpolydiethanolamide, stearylamine, stearyl propylenediamine, coconut acidamine, laurylamine, oleylamine, stearylamine, tallow acid amine, theethoxylates and/or propoxylates of said compounds, which may contain 1to 50, preferably 1 to 20 alkylene oxide units, also the esters of saidcompounds, such as coconut acid monoethanolamide acetate. Mixtures ofsaid compounds can also be used.

[0046] Coating agents II are preferably employed in a concentration of50 to 50000 ppm, particular preferably 100 to 5000 ppm, still morepreferably 300 to 3000 ppm with respect to component I.

[0047] In a particular embodiment of the present invention, at least oneLewis acid is added as coating agent III to the polymers in addition toat least one coating agent II,. In accordance with the invention,electron pair acceptors can be employed as the Lewis acid.

[0048] The compounds that can be used as Lewis acid III or coating agentIII in the absorption medium of the invention are inorganic acids,water-soluble, saturated or unsaturated organic acids, water-solublehydrocarbon acids or water-soluble acid salts.

[0049] The Lewis acids are preferably inorganic acids such as hydrogenhalides, oxyhalogen acids, sulphur or selenium oxyacids, nitrogen orphosphorus oxyacids, organic acids such as water-soluble saturated orunsaturated organic acids, and/or water-soluble acid salts such aswater-soluble bromides, chlorides, nitrates, sulphates, phosphates orsalts of organic acids such as the acetate, formate, oxalate and lactateof the metals Al, Fe, Zn, Sb, As, Sn, Cu, Mg, Ca, Cr, Ga, V, Ti, Bi, Tl,In, Mn, Ni, Co, Be und zirconium.

[0050] Preferably, the inorganic acids are hydrochloric acid, perchloricacid, bromic acid, hydrobromic acid, sulphuric acid, sulphur-containingacids, selenic acid, nitric acid, phosphonic acid orphosphorus-containing acids; the organic water-soluble acids arepreferably carbonic acid, hydrocarbon acids, sulphonic acids orphosphonic acids or the corresponding amino acids, for example acrylicacid, methacrylic acid, formic acid, acetic acid, propionic acid,butyric acid, oxalic acid, malonic acid, succinic acid, lactic acid,maleic acid, fumaric acid, benzoic acid, phthalic acid, salicylic acid,tartaric acid, citric acid, p-, m- und o-toluenesulphonic acid,benzenesulphonic acid, aminomethanesulphonic acid,aminomethanephosphonic acid; and the acid salts are aluminium salts,alums and their various hydrates such as AlCl₃×6H₂O, NaAl(SO₄)₂×12H₂O,KAl(SO₄)₂×12H₂O or Al₂(SO₄)₃×14-18H₂O, zinc salts and their hydratessuch as ZnCl₂, ZnSO₄×1-7H₂O and Zn(CH₃COO)₂×2H₂O, iron salts and theirhydrates such as NaFe(SO₄)₂×12H₂O, KFe(SO₄)₂×12H₂O and Fe₂(SO₄)₃×n H₂O,Mg-salts such as MgCl₂ or MgSO₄, double salts, also mixtures of thesalts and mixtures of the inorganic and/or organic acids and mixtures ofthe salts with the inorganic and/or organic acids.

[0051] Particularly preferred compounds are: for the inorganic acids,sulphuric acid or phosphoric acid; for the acid salts: AlCl₃×6H₂O,Al₂(SO₄)₃×14-18H₂O, ZnCl₂, ZnSO₄×1-7H₂O, Zn (CH₃ COO)₂×2H₂O, MgSO₄,MgCl₂; and for the organic acids, acetic acid, oxalic acid, lactic acid,citric acid and tartaric acid.

[0052] Particularly preferably, the Lewis acid is sulphuric acid,phosphoric acid, formic acid, acetic acid, citric acid orp-toluenesulphonic acid, an aluminium salt or alum and/or their varioushydrates, zinc salts and/or hydrates thereof, magnesium salts and/orhydrates thereof and/or double salts.

[0053] In accordance with the invention, at least one Lewis acid isused. However, a mixture of at least two of the cited Lewis acids can beused.

[0054] The total amount of coating agents II and III is 100 to 50000ppm, preferably 300 to 25000 ppm, more particularly 500 to 13000 ppmwith respect to component I.

[0055] Coating agent II can be added with Lewis acid III or before orafter carrying out the surface cross-linking step or simultaneously withthe cross-linking agent, and then undergo the heat treatment withpolymer I. Alternatively, when a combination of coating agent II withLewis acid III is used, the two compounds can be added separately,preferably as aqueous solutions, or they can be added simultaneously topolymer I, optionally together with the cross-linking agent, preferablyas an aqueous solution. Preferably, coating agent II, optionally incombination with Lewis acid III, is added simultaneously with thecross-linking agent and the coated polymer then undergoes heattreatment, thus dispensing with an additional process step. Particularlypreferably, component I is coated with an aqueous solution of coatingagent II and III and then reacted. Still more preferably, coating agentII is a fatty acid alkanolamide or a fatty acid amine with formula I,which may be alkoxylated and/or esterified.

[0056] Suitable solvents are water or polar, water-miscible organicsolvents such as acetone, methanol, ethanol or 2-propanol or mixturesthereof, preferably water. The term “aqueous solution” as used in thecontext of the invention means, when referring to solvent components,that they can also contain organic solvents in addition to water. Theconcentration of the optional cross-linking agent in the coatingsolution can vary between wide limits and is primarily in the range 1%to 80% by weight, preferably in the range 1% to 60% by weight,particularly preferably in the range 10% to 50% by weight. Theconcentration of coating agent II or optional Lewis acid III in thesolution can also vary between wide limits, preferably in the range 0.5%to 80% by weight, preferably in the range 0.5% to 60% by weight,particularly preferably in the range 0.5% to 60% by weight andparticularly preferably in the range 0.5% to 30% by weight. Thepreferred solvent for the optional organic cross-linking agent andcoating agent II and optional Lewis acid III is water, preferably in anamount of 0.5% to 10% by weight, particularly preferably 0.5% to 5% byweight and still more preferably 0.5% to 4% by weight with respect topolymer I.

[0057] To obtain the desired properties, the coating solution(s) must beevenly distributed on the absorbent polymer. To this end, the componentsare thoroughly and homogeneously mixed in suitable mixers, such asfluidised bed mixers, pan mixers, roller mixers or twin screw mixers.

[0058] It is also possible to coat the polymers during production of thepolymer, preferably at the final stage of polymerization. To this end,reverse suspension polymerization is suitable.

[0059] Heat treatment of component I coated with coating agent II or acorresponding coating solution is preferably carried out at temperaturesof 100° C. to 250° C., particularly preferably 150° C. to 230° C., moreparticularly preferably 150° C. to 210° C., to cause component I toreact with the coating agent.

[0060] If coating agent II and III or the corresponding coating solutionis used, heat treatment is preferably carried out at a temperature of40° C. to 250° C., particularly preferably 100° C. to 230° C., moreparticularly preferably 130° C. to 210° C., to cause component I toreact with the coating agent.

[0061] Preferably, coating agent II is added to the absorbent polymer incombination with a Lewis acid III before, after or simultaneously withthe cross-linking agent prior to heat treatment, as in this case thetemperature and duration of the heat treatment is lower and shorter thanwhen treating the absorbent polymer with coating agent II without Lewisacid III.

[0062] The particle size of the powder to be coated is preferably in therange 50 to 5000 μm, particularly preferably in the range 50 to 1000 μmand more particularly preferably in the range 50 to 850 μm. The particlesize is determined using known screening methods.

[0063] The heat treatment period is also dependent on the selectedtemperature. It should be noted in this regard that the higher thetemperature, the shorter the period. The treatment time and temperatureare selected so that the treated polymer has a reduced caking tendency,i.e., it passes the anti-caking test (≧3 hours), retaining or improvingthe retention, absorption under load and uptake rate for water oraqueous fluids, in particular body fluids, compared with superabsorbentpolymers that are not in accordance with the invention. For polymersbased on partially neutralized and cross-linked poly(meth)acrylic acids,this means a retention of ≧20 g/g and an AUL_(0.9) of ≧19 g/g using themethods described below.

[0064] The invention also concerns absorbent agent produced by theprocess of the invention.

[0065] The polymers treated in accordance with the invention are easy tomanipulate, for example easy to convey and dose.

[0066] The polymers of the invention or absorbent agents are preferablyused in absorbent hygiene products such as diapers, incontinenceproducts for adults and feminine pads.

[0067] Absorbent hygiene products usually have a general constructionconstituted by a fluid-permeable cover facing the body, afluid-absorbing absorbent layer and an essentially fluid impermeable,outer layer facing away from the body. Optionally, they may have furthermeans for rapid uptake and distribution of body fluids to the absorbentcore. These constructions are often, but not necessarily between thefluid-permeable cover facing the body and the fluid-absorbing absorbentcore.

[0068] The fluid-permeable cover usually consists of a nonwoven, fibrousfabric or a different porous construction.

[0069] Examples of materials for this cover are synthetic polymers suchas polyvinyl chloride or fluoride, polytetrafluorethylene (PTFE),polyvinylalcohols and their derivatives, polyacrylates, polyamides,polyesters, polyurethanes, polystyrene, polysiloxane or polyolefins (forexample polyethylene (PE) or polypropylene (PP)), also natural fibrousmaterials and combinations of the above materials as mixed materials orcomposites or copolymers.

[0070] The fluid permable cover is hydrophilic in nature. It can alsoconsist of a combination of hydrophilic and hydrophobic components.Preferably, the fluid-permeable cover has a hydrophilic framework sothat body fluids can trickle quickly through into the fluid-absorbingabsorbent layer, however partially hydrophobic covers are also used.

[0071] Fluid-Absorbing Absorbent Layer

[0072] The fluid-absorbing absorbent layer contains the superabsorbentpowdered or granulated polymer of the invention and further components,for example fibrous materials, foamed materials, film-forming materialsor porous materials as well as combinations of two or more of suchmaterials. Each material can be natural or synthetic in origin or can beproduced by chemical or physical modification of natural materials. Thematerials can be hydrophilic or hydrophobic, preferably hydrophilic.This is particularly the case for compositions that efficiently take upexuded body fluids and transport the body fluid to a location in theabsorbent core that is at a distance from the entry point.

[0073] Suitable fibrous materials are cellulose fibres, modifiedcellulose fibres (for example stiffened cellulose fibres), polyesterfibres (for example Dacron), hydrophilic nylon or hydrophilisedhydrophobic fibres, such as polyolefins (PE, PP) hydrophilised withsurfactants, polyesters, polyacrylates, polyamides, polystyrene,polyurethanes and the like.

[0074] Preferably, cellulose fibres and modified cellulose fibres areused. Combinations of cellulose fibres and/or modified cellulose fibreswith synthetic fibres such as PE/PP composite materials, termedbi-component fibres, such as those used for thremobonding of air laidmaterials, or other materials can also be used.

[0075] The fibrous materials can be in different forms, for exampleloose from an air stream or as an aqueous phase or deposited cellulosefibres, as a nonwoven fabric or as a tissue. Combinations of differentforms are possible.

[0076] Optionally, in addition to the superabsorber of the invention,other powdered substances can be used, for example odour-bindingsubstances such as cyclodextrin, zeolites, inorganic or organic saltsand the like.

[0077] The porous materials and foamed materials can, for example, bepolymer foams such as those described in DE 44 18 319 A1 and DE 195 05709 A1, hereby incorporated by reference and considered to constitutepart of the disclosure.

[0078] Thermoplastic fibres (for example bi-component fibres ofpolyolefins, polyolefin granulates, latex dispersions or hot meltadhesives) can be used to stabilise the fluid-absorbing absorbent layermechanically. Optionally, one or more layers of tissue can be used forstabilisation.

[0079] The fluid-absorbing absorbent layer can be a single layer, or itcan consist of a plurality of layers. Preferably, constructions are usedthat consist of hydrophilic fibres, preferably cellulose fibres,optionally a construction for rapid uptake and distribution of bodyfluids, such as chemically stiffened (modified) cellulose fibres or highloft fabric from hydrophilic or hydrophilised fibres, and superabsorbentpolymers.

[0080] The superabsorbent polymer of the invention can be homogeneouslydistributed in the cellulose fibres or stiffened cellulose fibres, itcan be layered between the cellulose fibres or stiffened cellulosefibres, or the concentration of superabsorbent polymer can be graduatedwithin the cellulose fibres or stiffened cellulose fibres. The ratio ofthe total amount of superabsorbent polymer to the total amount ofcellulose fibres or stiffened cellulose fibres in the absorbent core canbe between 0 to 100 and 80 to 20, while in one embodiment, for examplethat with a gradient or layered structure, local concentrations of up to100% by weight of polymer can be obtained. Such constructions withregions of high concentrations of absorbent polymer, where theproportion of superabsorber in certain regions is between 60% and 100%by weight, or between 90% and 100% by weight at its highest, each basedon the total weight of the construction, are described, for example, inU.S. Pat. No. 5,669,894 A, hereby incorporated by reference andconsidered to constitute part of the disclosure.

[0081] Optionally, a plurality of different absorbent polymersdiffering, for example, in absorbing speed, permeability, storagecapacity, absorption under load, grain size distribution or chemicalcomposition, can be used at the same time. These different polymers canbe mixed together in the absorbent pad or can be in different locationsin the absorbent core. Such differentiated positioning can be along thethickness of the absorbent pad or along the length or breadth of theabsorbent pad.

[0082] One or more of the layers of cellulose fibres or stiffenedcellulose fibres containing superabsorbent polymers are contained in theabsorbing absorbent layer. In a preferred embodiment, constructions ofcombinations of layers with homogeneous superabsorbing layers andadditional layers are used.

[0083] Optionally, the cited structures can be supplemented by furtherlayers of pure cellulose fibres or stiffened cellulose fibres on theside facing the body and/or facing away from the body.

[0084] The constructions described above can be repeated a plurality oftimes, by stacking two or more similar layers or by stacking two or moredifferent constructions. The differences may be entirely constructive ormay reside in the type of the materials used, such as the use ofabsorbent polymers with different properties or different cellulosetypes.

[0085] Optionally, the entire absorbent pad or individual layers of theabsorbing absorbent layer can be separated from the other componentswith layers of tissue or they may be in direct contact with other layersor components.

[0086] As an example, the structure for rapid uptake and distribution ofbody fluids can be separated from the absorbing absorbent layer bytissue or they can be in direct contact with each other. If no separateconstruction for rapid uptake and distribution of body fluid existsbetween the absorbing absorbent layer and the fluid-permeable coverfacing the body, but the effect of fluid distribution is to be obtained,for example by using a special fluid-permeable cover on the body side,the fluid-permeable cover facing the body can optionally be separatedfrom the fluid-absorbing absorbent layer by a tissue.

[0087] Optionally, instead of tissue, a nonwoven fabric can be added tothe fluid-absorbing absorbent layer. Both components result in thedesired side effect of stabilising and fixing the absorbent core whenmoist.

[0088] Process for Producing the Fluid-Absorbing Absorbent Layer

[0089] Fibre-containing, superabsorber-containing, fluid distributingand storing layers can be generated by a plurality of processes.

[0090] Established conventional processes, summarised by the skilledperson as drum forming with the assistance of shaping wheels, pocketsand product shapes and suitable corresponding dosing apparatus for theraw materials, are supplemented by modem processes such as the air laidprocess (e.g. EP 850 615, cn. 4 line 39 to cn. 5 line 29, U.S. Pat. No.4,640,810) with all types of dosing, deposition of fibres and fixingsuch as hydrogen bonding (e.g. DE 197 50 890, cn. 1 line 45 to cn. 3line 50, thermo bonding, latex bonding (e.g. EP 850 615, cn. 8 line 33to cn. 9 line 17 and hybrid bonding, the wet laid process (e.g. PCT WO99/49905, cn. 4 line 14 to cn. 7 line 16), carding, melt blown, spunblown processes and similar processes for the production ofsuperabsorber-containing nonwovens (as defined by EDANA, Brussels), alsocombinations of these processes with other normal methods for theproduction of the cited fluid storage means. The documents cited aboveare hereby incorporated by reference and should be considered toconstitute part of the disclosure.

[0091] Further processes that can be considered are the production oflaminates in the broadest sense and the production of extruded andco-extruded, wet and dry structures and post-formed structures.

[0092] A combination of these processes with each other is alsopossible.

[0093] Constructions for Rapid Uptake and Distribution of Body Fluids

[0094] A construction for rapid uptake and distribution of body fluidsconsists, for example, of chemically stiffened (modified) cellulosefibres or high loft fabrics of hydrophilic or hydrophilised fibres or acombination of the two.

[0095] Chemically stiffened, modified cellulose fibres can, for example,be produced from cellulose fibres, which are chemically transformed bycross-linking agents such as C₂-C₈ dialdehydes, C₂-C₈ monoaldehydes withan additional acid function, or C₂-C₉ polycarbon acids. Particularexamples are: glutaraldehyde, glyoxal, glyoxalic acid or citric acid.Cationically modified starches or polyamide-epichlorhydrin resins (forexample KYMENE 557H, Hercules Inc., Wilmington, Del.) are also known.Cross-linking produces a twisted, crumpled structure that is stable,which advantageously affects the rate of fluid uptake.

[0096] Weight Per Unit Area and Density of Absorbent Articles

[0097] The absorbent hygiene products can vary widely in weight per unitarea and thickness and thus in density. Typically, the density of theabsorbent core is in the range 0.08 to 0.25 g/cm³. The weight per unitarea is between 10 and 1000 g/m², and preferably the weight per unitarea is between 100 and 600 g/m² (see also U.S. Pat. No. 5,669,894,hereby incorporated by reference and considered to constitute part ofthe disclosure). The density normally varies over the length of theabsorbent core. This is as a result of predetermined dosing of thecellulose fibre or stiffened cellulose fibre or the amount of thesuperabsorbent polymer, as in preferred embodiments, these componentsare more concentrated in the frontal area of the absorbent disposablearticles.

[0098] This deliberate increase in the concentration of absorbentmaterial in particular regions of the absorbent core can be achieved inother ways, for example by producing an appropriately dimensioned flatform using an air laid or wet laid process consisting of hydrophiliccellulose fibres, optionally from stiffened cellulose fibres, optionallyfrom synthetic fibres (for example polyolefins) and superabsorbentpolymers and then folding it back or stacking.

[0099] Test Methods

[0100] Unless otherwise indicated, the following tests are carried outusing polymers with a particle size of 300 to 600 μm (determined usingthe screen method).

[0101] Anti-Caking Test

[0102] 5 g±0.1 of polymer with a particle size of 150 to 180 μm isweighed into an aluminium weighing boat (57 mm) and distributedhomogeneously over the entire boat. The boat is weighed. Then the boatis placed in a heated cabinet at a temperature of 35° C. with a relativehumidity of 80% for 3, 6 or 24 h. Then the boat is weighed again. Afurther boat is weighed and a sieve with a mesh of 1.5 mm is placed overit. The sample is tipped onto the sieve. After tapping lightly on thesieve a number of times, the weight of the product that has fallenthrough the sieve is measured.

[0103] The test is considered to have been passed when more than 90% byweight of the product falls through the sieve. The water uptake of theproduct is also determined.

[0104] Method for Measuring Surface Tension:

[0105] Measurement of surface tension of aqueous solutions using aTraube-Gerhardt stalagmometer.

[0106] A stalagmometer is a type of volumetric pipette that empties intoa very carefully produced drip surface. On this polished surface,droplets form one after the other, the size of which is dependent on thesurface tension of the product under consideration. The higher thesurface tension, the larger the droplets, and vice versa. The volume ofthe pipette is calibrated with circular marks. Since the volume isconstant and the droplet size is dependent on the surface tension, thenumber of droplets is a direct measure of the surface tension. The valuethat is measured, therefore, is compared with the number of droplets ofpure water, the surface tension of which is known.

[0107] 150 g of 0.9% NaCl solution is placed in a 250 ml beaker andstirred with a magnetic stirrer (200 rpm). 1 g of the test polymer isslowly scattered in the spout formed by the 0.9% NaCl solution. Whenscattering is complete, the solution is stirred for 3 minutes. It isthen allowed to stand for 15 minutes.

[0108] The test solution is drawn up to well beyond the upper pipettevolume mark using a pipette bulb. The number of droplets between theupper and lower mark are counted. Each solution is tested twice.

[0109] Calculation of surface tension in mN/m=

[0110] Number of droplets of pure water×72.75*/test sample dropletnumber *(surface tension of water in mN/m).

[0111] Measuring the surface tension establishes how much coating agentmay be released into an aqueous environment. In other words, thismeasurement establishes how well the coating agent is bonded to thepolymer. If the surface tension is reduced by the coating, then theproblem of re-wetting can occur. Re-wetting causes fluids, for exampleurine, that have already been absorbed to be released, for example, bypressure on the swollen gel, meaning that the hygiene article is notcomfortable for the wearer.

[0112] Retention:

[0113] The retention is obtained using the teabag method and the averageof three measurements is taken. About 200 mg of polymer is sealed in ateabag and soaked in 0.9% NaCl solution for 30 minutes. Then the teabagis centrifuged in a centrifuge (23 cm diameter, 1400 rpm) for 3 minutesand then weighed. A teabag with no absorbent polymer is run at the sametime as a reference. Retention [g/g]=endothermic weight−referenceweight/starting weight

[0114] Fluid Uptake Under 0.9 psi Load, AUL:

[0115] 0.16 g of polymer is accurately weighed into a Plexiglas cylinderwith an internal diameter of 25.4 mm fitted with a 400 mesh nylon sievebase. The layer of polymer evenly distributed on the sieve base iscovered with a 26.1 mm diameter Teflon disk and weighed down with acylindrical piston weighing 332.4 g. The Teflon disk and piston togetherconstitute a load of 63 g/cm², or 0.9 psi. The weighed cylinder is thenplaced on a glass filter plate in a dish with 0.9% NaCl solution thedepth of which exactly corresponds to the depth of the filter plate.After the cylinder assembly has been left for 1 hour to allow the 0.9%NaCl solution to be absorbed, the filter paper is patted free of excesstest solution and then re-weighed and the AUL is calculated as follows:

[0116] AUL=final weight (cylinder assembly+swollen polymer)−start weight(cylinder assembly+non swollen polymer)/ polymer start weight

EXAMPLES

[0117] The invention will now be illustrated by examples. The examplesare given purely by way of illustration and in no way limit the scope ofthe invention.

[0118] The following abbreviations are used: ABAH2.2′-azo-bis-amidinopropane-dihydrochloride AIBN2.2′-azo-bis-2-methylpropionitrile AMPS2-acrylamido-2-methylpropanesulphonic acid BO 2-butyl-octanol EOethylene oxide (1,2-epoxyethane) IHD isohexadecane ITDAisotridecylalcohol ITS isotridecylstearate DN degree of neutralisation,mole-% OABOE oleic acid butyloctyl ester ROSME rapeseed oil acid methylester TAMAC triallylmethylammonium chloride

Comparative Example 1 U.S. Pat. No. 5,728,742

[0119] 1000 ppm of Ethoquad 0/12 dissolved in 3 g of isopropanol wasadded, using a syringe, stirring with a mixer, to 50 g of powderedpolyacrylate that had been 70% neutralized and surface cross-linked(Favor SXM 880®, available from Stockhausen GmbH & Co. KG), with aretention of 32 g/g in 0.9% NaCl-solution and a AUL_(0.9 psi) of 22.1g/g. The polymer was rolled for 60 minutes on a roll bench at ambienttemperature. The product did not pass the 3 h anti-caking test describedabove.

[0120] Ethoquad 0/12=oleylmethyldi(2-hydroxyethyl)ammonium chloride

Comparative Example 2 U.S. Pat. No. 5,728,742

[0121] 1000 ppm of Arquad 16-50 dissolved in 3 g of isopropanol wasadded, using a syringe, stirring with a mixer, to 50 g of powderedpolyacrylate as described in comparative example 1 (Favor SXM 880®,available from Stockhausen GmbH & Co. KG). The polymer was rolled for 60minutes on a roll bench at ambient temperature. The product did not passthe 3 h anti-caking test described above.

[0122] Arquad 16-50=Hexadecyltrimethylammonium chloride

[0123] Production of polymer powders 1-5

[0124] Powder 1

[0125] 290 g of acrylic acid was divided into two equal portions. Oneportion was added to 458.5 g of H₂O. 0.85 g of polyethyleneglycol-300-diacrylate and 1.5 g of allyloxypolyethylene glycol acrylicacid ester were dissolved in the second portion of acrylic acid and thenadded to the water. The solution was cooled to 10° C., then a total of225.4 g of 50% sodium hydroxide was slowly added with cooling so thatthe temperature did not exceed 30° C. The solution was then flushed withnitrogen at 20° C. and then cooled again. When the start temperature of4° C. had been reached, the initiator solutions (0.1 g of2.2′-azobis-2-amidinopropane-dihydrochloride in 10 g H₂O, 1.0 g sodiumperoxydisulphate in 10 g H₂O, 0.1 g 30% hydrogen peroxide solution in 1g H₂O and 0.015 g ascorbic acid in 2 g water) was added. After the finaltemperature of 102° C. had been reached, the gel that had formed wasdisintegrated and then dried for 90 minutes at 150° C. the dried polymerwas coarsely crushed, ground and screened to a powder with a particlesize of 150 to 850 μm.

[0126] Powder 2

[0127] 300 g of acrylic acid was divided into two equal portions. Oneportion was added to 429.1 g of H₂O. 0.36 g of triallylamine, 1.05 g ofallyloxypolyethylene glycol acrylic acid ester and 12 g ofmethoxypolyethylene glycol (22EO) methacrylate were dissolved in thesecond portion of acrylic acid and then added to the water. The solutionwas cooled to 10° C. Then a total of 233.1 g of 50% sodium hydroxide wasslowly added with cooling so that the temperature did not exceed 30° C.The solution was then flushed with nitrogen at 20° C. and then cooledagain. When the start temperature of 4° C. had been reached, 0.9 g ofsodium carbonate and the initiator solutions (0.1 g of2.2′-azobis-2-amidinopropane-dihydrochloride in 10 g H₂O, 0.15 g ofsodium peroxydisulphate in 10 g H₂O, 0.1 g of 30% hydrogen peroxidesolution in 1 g H₂O and 0.01 g ascorbic acid in 2 g water) were added.After the final temperature of 104° C. had been reached, the gel thathad formed was disintegrated and then dried for 90 minutes at 150° C.The dried polymer was coarsely crushed, ground and screened to a powderwith a particle size of 150 to 850 μm.

[0128] Powder 3:

[0129] was a non surface cross-linked polyacrylic acid (fabricationproduct of Favor SXM 880®), 70% neutralized and with a retention of 40g/g in 0.9% NaCl solution and an AUL of 8.7 g/g.

[0130] Powder 4:

[0131] 280 g of acrylic acid was divided into two equal portions. Oneportion was added to 517.04 g of H₂O. 0.28 g of triallylamine, 0.72 g ofallyloxypolyethylene glycol acrylic acid ester and 7.51 g ofmethoxypolyethylene glycol (22EO) methacrylate were dissolved in thesecond portion of acrylic acid and then added to the water. The solutionwas cooled to 10° C. Then a total of 170.97 g of 50% sodium hydroxidewas slowly added with cooling so that the temperature did not exceed 30°C. The solution was then flushed with nitrogen at 20° C. and then cooledagain. When the start temperature of 4° C. had been reached, 0.9 g ofsodium carbonate and the initiator solutions (0.1 g of2.2′-azobis-2-amidinopropane-dihydrochloride in 10 g of H₂O, 1.0 g ofsodium peroxydisulphate in 10 g H₂O, 0.07 g of 30% hydrogen peroxidesolution in 1 g H₂O and 0.01 g ascorbic acid in 2 g water) were added.After the final temperature of 102° C. had been reached, the gel thathad formed was disintegrated and then dried for 90 minutes at 150° C.The dried polymer was coarsely crushed, ground and screened to a powderwith a particle size of 150 to 850 μm.

[0132] Powder 5:

[0133] was the non surface cross-linked polyacrylic acid of comparativeExample 1 with a retention of 32 g/g in 0.9% NaCl solution and an AUL of22.1 g/g (Favor SXM 880®).

Example 1

[0134] A mixture of 0.015 g of Comperlan COD, 0.25 g of ethylenecarbonate as a cross-linking agent, 1.0 g of H₂O and 4.0 g of acetonewas added to 50 g of powder 1, stirring with a mixer (Krups Dry Mix Type7007) on its highest setting. The coated polymer was evenly poured intoa photographic tank and dried for 30 minutes in a circulating air dryingoven at 180° C. The retention, AUL value, surface tension andanti-caking tests were carried out as described above and the resultsare shown in Table 1.

Examples 2-17

[0135] Examples 2-17 were carried out as described for Example 1. Thepowder used, added compounds, the amounts of the compounds and theduration and temperature of the heat treatment are shown in Table Ialong with the corresponding retention, AUL, surface tension and theresults of the anti-caking tests.

Comparative Example 3

[0136] A mixture of 0.05 g of Imbentin CMEA/045 in 1.0 g of H₂O wasadded to 50 g of powder 5, stirring with a mixer (Krups Dry Mix Type7007) on its highest setting; it was then stirred for a further 2minutes. No heat treatment was carried out. The polymer did not pass the3 h anti-caking test and had reduced pourability.

[0137] The polymer had a retention of 31.4 g/g and an AUL of 21.3 g/g.

Example 18

[0138] A mixture of 0.05 g of Imbertin CMEA/045 in 1.0 g of H₂O wasadded to 50 g of powder 5, stirring with a mixer (Krups Dry Mix Type7007) on its highest setting. It was then stirred for a further 2minutes. The product was poured into a photographic tank and dried for20 minutes in a circulating air drying oven at 180° C. The treatedpolymer passed the anti-caking test. Its retention, AUL and surfacetension are shown in Table 1.

Example 19

[0139] A mixture of 0.05 g of Imbentin CMEA/045 in 1.0 g of H₂O wasadded to 50 g of powder 5, stirring with a mixer (Krups Dry Mix Type7007) on its highest setting. It was then stirred for a further 2minutes. The product was poured into a photographic tank and dried for15 minutes in a circulating air drying oven at 190° C. The treatedpolymer passed the anti-caking test. Its retention, AUL and surfacetension are shown in Table 1. TABLE 1 Conc'n, wt % 3 h with Temperature/anti- AUL Surface Anti- respect to time caking Retention 0.9 psi tensionExample Powder caking substance product [° C./t] test [g/g] [g/g] [mN/m]1 — 180/30 − 31.5 17.7 1 1 Comperlan 0.015 180/30 + 31.5 17.6 72.5 COD 21 Comperlan 0.06 180/30 + 32.8 17.4 72.5 LD 2 — 180/30 − 34.8 23.5 — 3 2Marlazin 0.03 180/40 + 35.0 23.7 72.5 L10 4 2 Marlazin 0.10 180/40 +34.9 23.8 72.5 L10 5 2 Serdox 0.10 180/30 + 34.5 22.5 72.5 NXC 3 6 2Serdox 0.10 180/40 + 35.0 21.0 72.5 NXC 6 7 2 Serdox 0.10 180/40 + 34.822.0 72.5 NXC 14 8 2 Serdox 0.05 180/40 + 35.2 22.3 72.5 NXC 6 3 —190/25 − 32.4 21.2 — 9 3 Serdox 0.10 180/35 + 32.3 19.1 72.5 NXC 6 10 3Marlazin 0.10 180/35 + 32.3 20.3 72.5 OL 20 11 3 Marlazin L10 0.10180/35 + 32.3 19.5 72.5 12 3 Comperlan 0.2 180/30 + 31.3 21.1 72.5 COD13 3 Serdox 0.1 180/30 + 31.7 21.2 72.5 NXC 6 14 3 Imbentin 0.1 190/20 +32.1 21.0 72.5 CMEA/045 15 3 Imbentin 0.2 190/20 + 32.0 20.8 72.5CMEA/045 4 — 170/25 − 30.5 21.0 — 16 4 Stokomin 0.1 170/25 + 30.1 21.272.5 S10 17 4 Serdox 0.1 170/25 + 30.9 20.4 72.5 NXC 3 5 — 32.0 22.1 185 Imbentin 0.05 180/20 + 30.9 20.8 72.5 CMEA/045 19 5 Imbentin 0.05190/15 + 30.8 21.0 72.5 CMEA/045

Comparative Example 4

[0140] 0.5 g of ethylene carbonate, 2 g of water and 0.25 g ofAl₂(SO₄)₃×18H₂O were mixed together and added using a syringe to 50 g ofpowder 3, stirring with a mixer (Krups Dry Mix Type 7007) on its highestsetting. The product was poured into a photographic tank and dried for50 minutes in a circulating air drying oven at 170° C.

[0141] The treated polymer failed the 3 h anti-caking test. Itsretention, AUL and surface tension are shown in Table 2.

Example 20

[0142] Powder 3 was mixed with 0.7% by weight of ethylene carbonate,1.8% by weight of water, 0.2% by weight of Al₂(SO₄)₃×14H₂O and 0.2% byweight of Imbentin CMEA/024 (Kolb AG) in a mixer (Krups Dry Mix Type7007) on its highest setting, and coated. Then, the treated polymer wasplaced in a blade dryer and left for 10 minutes at a temperature of 110°C.

[0143] The product passed the 3 h anti-caking test. Its retention andAUL are shown in Table 2.

Example 21

[0144] 0.5 g of ethylene carbonate, 0.5 g of acetone, 2 g of water,0.075 g of ZnCl₂ and 0.05 g of Imbentin CMEA/045 (Kolb AG) were mixedand added to 50 g of powder 3 using a syringe, stirring with a mixer(Krups Dry Mix Type 7007) on its highest setting. Then, the polymer waspoured into a photographic tank and dried for 50 minutes in acirculating air drying oven at 170° C.

[0145] The treated polymer passed the 3 h anti-caking test. Itsretention, AUL and surface tension are shown in Table 2.

Example 22

[0146] Powder 3 was coated by adding a mixture of 2% by weight of waterand 1% by weight of H₂SO₄ (98%) with a syringe, stirring using a mixer.0.5 g of ethylene carbonate, 0.5 g of acetone, 0.05 g of ImbentinCMEA/045 and 1 g of water were mixed together and added using a syringeto 50 g of powder 3 stirring with the mixer (Krups Dry Mix Type 7007) onits highest setting. Then, the polymer was poured into a photographictank and dried for 50 minutes in a circulating air drying oven at 170°C.

[0147] The treated polymer passed the 3 h anti-caking test. Itsretention, AUL and surface tension are shown in Table 2.

Example 23

[0148] 0.5 g of ethylene carbonate, 0.5 g of acetone, 1 g of water, 1 gof H₃PO₄ (85%) and 0.1 g of Imbentin CMEA/045 (Kolb AG) were mixedtogether and added using a syringe to 50 g of powder 3 stirring with themixer (Krups Dry Mix Type 7007) on its highest setting. Then, thepolymer was poured into a photographic tank and dried for 50 minutes ina circulating air drying oven at 170° C.

[0149] The treated polymer passed the 3 h anti-caking test. Itsretention, AUL and surface tension are shown in Table 2.

Examples 24-27

[0150] Examples 24-29 were carried out as described for Example 21, withthe amounts of Lewis acids and Imbentin CMEA/045 employed shown in Table2. The results of the 3 h anti-caking tests, retention, AUL and surfacetension for the respective polymers are shown in Table 2. TABLE 2Imbentin*/ Temp./ 3 h Lewis acid** time Retention AUL_(0.9 psi) Anti-Example Lewis acid (Gew. %) (° C./min) (g/g) (g/g) caking Comp.Al₂(SO₄)₃ × 18H₂O 0.0/0.50 170/50 31.5 22.6 − Ex. 4 20 Al₂(SO₄)₃ × 14H₂O0.2/0.20 110/10 35.7 7.2 + 21 ZnCl₂ 0.1/0.15 170/50 32.8 21.8 + 22 H₂SO₄0.1/1.00 170/50 31.9 19.0 + 23 H₃PO₄ 0.2/2.00 170/50 30.0 17.1 + 24FeCl₃ × 6H₂O 0.1/0.05 170/50 32.3 21.4 + 25 MgSO₄ × 7H₂O 0.3/0.10 170/5031.5 23.6 + 26 AlCl₃ × 6H₂O 0.1/0.05 160/50 35.9 14.0 + 27 FeCl₃ × 6H₂O0.2/0.10 170/50 32.1 19.2 +

Examples 28-29

[0151] 0.5 g of ethylene carbonate, 2 g of water, aluminium sulphate andImbentin CMEA/045 (Kolb AG) were mixed together and added using asyringe to 50 g of powder 3 stirring with the mixer (Krups Dry Mix Type7007) on its highest setting. Then, the polymer was poured into aphotographic tank and dried for 50 minutes in a circulating air dryingoven at 170° C. The reaction conditions and product properties are shownin Table 3.

Examples 30-33

[0152] Two polyacrylic acids were produced with a degree ofneutralisation of 65% or 70% and a retention of 34% or 32 g/g in 0.9%aqueous NaCl solution as described for the production of powder 1. 50 gof this powder (particle size in the range 150 to 850 μm) was mixed witha mixture consisting of 0.5 g of ethylene carbonate, 2 g of water,aluminium sulphate×14H₂O and Imbentin CMEA/045 using a mixer (Krups DryMix Type 7007) on its highest setting. The powder was then dried in acirculating air drying oven in a photographic tank. The reactionconditions and properties of the products are shown in Table 3. TABLE 3Start Anti- AUL product Retention Temp. Time caking 0.9 psi Ex. DN g/gImbentin⁽¹⁾ Al₂(SO₄)₃ × 14H₂O⁽¹⁾ ° C. min test⁽²⁾ Retention g/g 28 70 390.1 0.1 170 60 6 31.7 22.0 29 70 39 0.1 0.2 170 60 6 31.6 21.5 30 65 340.25 0.45 170 60 24 30 20.5 31 65 34 0.25 0.50 170 60 24 30 21 32 70 320.30 0.50 180 30 24 28 19.5 33 70 32 0.25 0.60 180 30 24 30 20

1. A highly swellable absorption medium with a reduced caking tendencyin a moist environment and/or at high temperatures comprising: I awater- or aqueous fluid-absorbing natural polymer modified with acidgroups or a water-insoluble, optionally surface cross-linked, water- oraqueous fluid-absorbing cross-linked polymer based on polymerizedmonomers comprising at least partially neutralized acid groups, which istreated with: II at least one coating agent selected from the groupconsisting of nitrogen-containing, non-ionic surfactants; wherein themixture formed from components I and II has been heat treated.
 2. Theabsorption medium according to claim 1, wherein the surfactant is atleast one compound with general formula I:

in which R₁ is a z-substituted aliphatic residue, preferably az-substituted, saturated or unsaturated, linear or branched aliphatic C₁to C₃₀ hydrocarbon residue, more preferably C₈ to C₂₂, which optionallycarries aryl residues, preferably a phenyl residue, a z-substitutedbenzene residue, optionally condensed with five or six-membered ringsoptionally containing heteroatoms such as oxygen, phosphorus ornitrogen; R₂ is a hydrogen; or an aliphatic residue, preferably asaturated or unsaturated, linear or branched C₂ to C₂₄ hydrocarbonresidue, preferably C₈ to C₂₂; a hydroxyalkylene residue, the hydroxylgroup of said hydroxyalkylen residue is preferably an end group and/oroptionally alkoxylated with 1 to 50, preferably 1 to 20, moreparticularly with 1 to 10 alkylene oxide units, preferably ethyleneand/or propylene oxide units, and/or said hydroxyl group is optionallyesterified with a carbon acid, preferably a C₁- to C₈-carbon acid, andthe alkylene group of said hydroxyalkylen residue is a C₁- to C₈-,preferably C₁-to C₄-hydrocarbon group, occurs in the alkylene residue,or a N,N-dihydroxyalkylene-amino-alkylene residue with C₁-C₄ in eachalkylene residue; R₃, which may be identical or different, has the samemeaning as R₂, provided that with amide compounds, at least one ofresidues R₂ or R₃ represents a hydroxyalkylene residue or an alkoxylatedhydroxyalkylene residue or a corresponding esterified or alkoxylated oresterified and alkoxylated hydroxyalkylene residue with the definitiongiven for R₂ above; n is 0 or 1, preferably 1; and z is a whole numberfrom 1 to
 4. 3. The absorption medium according to claim 1, whereinpolymer I is surface cross-linked.
 4. The absorption medium according toclaim 1, wherein the coating agent II is used in an amount of 50 to50000 ppm, preferably 100 to 5000 ppm, particularly preferably 300 to3000 ppm.
 5. The absorption medium according to claim 1, wherein thecoated component I is heat treated at a temperature of 100° C. to 250°C., preferably 150° C. to 230° C., particularly preferably 160° C. to210° C.
 6. The absorption medium according to claim 1, wherein coatingagent II is a fatty acid alkanolamide or a fatty acid amine according toformula I, which is optionally alkoxylated and/or esterified.
 7. Theabsorption medium according to claim 1, wherein the coating agent II isat least one compound selected from the group consisting of lauric acidmonoethanolamide, coconut acid monoethanolamide, stearic acidmonoethanolamide, ricinic acid monoethanolamide, undecylenic acidmonoethanolamide, lauric acid diethanolamide, coconut aciddiethanolamide, soya acid diethanolamide, linoleic acid diethanolamide,laurylmyristic acid diethanolamide, oleic acid diethanolamide, lauricacid isopropanolamide, coconut acid isopropanolamide, oleic acidisopropanolamide, undecylenic acid polydiethanolamide, coconut acidpolydiethanolamide, stearylamine, stearyl propylenediamine, coconut acidamine, laurylamine, oleylamine, stearylamine, tallow fat amine; acorresponding ethoxylated and/or propoxylated compound with 1 to 50,preferably 1 to 20 ethylene oxide and/or propylene oxide units and thecorresponding esterified compound.
 8. The absorption medium according toclaim 1, wherein the component I to be coated is in the form of apowder.
 9. The absorption medium according to claim 1, wherein thepolymer is based on (meth)acrylic acid, the carboxyl groups of which areat least 50 mole % neutralized.
 10. Use of an absorption mediumaccording to claim 1 in composites for absorbing water and aqueous orserous fluids, preferably body fluids.
 11. Use of an absorption mediumaccording to claim 1 in hygiene articles, preferably diapers, tampons orfeminine pads.
 12. Use of an absorption medium according to claim 1 incable sheaths or packaging.
 13. Use of an absorption medium according toclaim 1 as a water storage means in floors or substrates.
 14. Use of anabsorption medium according to claim 1 to store nutrients and activeagents and for their controlled release.
 15. A highly swellableabsorption medium with a reduced caking tendency in a moist environmentand/or at high temperatures comprising: I a water- or aqueousfluid-absorbing natural polymer modified with acid groups or awater-insoluble, optionally surface cross-linked, water- or aqueousfluid-absorbing cross-linked polymer based on polymerized monomerscontaining at least partially neutralized acid groups, which is treatedwith: II at least one coating agent selected from the group consistingof nitrogen-containing, non-ionic surfactants; wherein the mixtureformed from components I and II has been heat treated; and wherein atleast one Lewis acid is used as a further coating agent III.
 16. Theabsorption medium according to claim 15, wherein the coating agents IIand III are used in a total amount of 100 to 50000 ppm, preferably 300to 25000 ppm, particularly preferably 500 to 13000 ppm.
 17. Theabsorption medium according to claim 15, wherein heat treatment of thecoated component I is carried out at a temperature of 40° C. to 250° C.,preferably 100° C. to 230° C., particularly preferably 130° C. to 210°C.
 18. The absorption medium according to claim 15, wherein component Iis coated with an aqueous solution of coating agents II and III andcaused to react.
 19. The absorption medium according to claim 15,wherein the Lewis acid III is an inorganic acid, a water-soluble,saturated or unsaturated organic acid, a water-soluble hydrocarbon acidor a water-soluble acid salt.
 20. The absorption medium according toclaim 19, wherein the Lewis acid is HCl, H₂SO₄, selenium or phosphorusoxyacids, HNO₃, H₂SO₂, H₂SO₃, HClO₃, HBr, acrylic acid, methacrylicacid, formic acid, acetic acid, propionic acid, butyric acid, oxalicacid, malonic acid, succinic acid, lactic acid, maleinic acid, fumaricacid, benzoic acid, phthalic acid, salicylic acid, tartaric acid, citricacid, p- m- or o- toluenesulphonic acid, benzenesulphonic acid,aminomethanesulphonic acid, aminomethanephosphonic acid, water-solublebromides, chlorides, nitrates, sulphates, phosphates, acetates,formates, oxalates or lactates of aluminium, iron, zinc, antimony,arsenic, tin, copper, magnesium, calcium, chromium, gallium, vanadium,titanium, bismuth, thallium, indium, manganese, nickel, cobalt,beryllium or zirconium or mixtures of two of more of said compounds. 21.The absorption medium according to claim 20, wherein the Lewis acid issulphuric acid, phosphoric acid, formic acid, acetic acid, citric acidor p-toluenesulphonic acid, aluminium salts or alums and/or hydratesthereof, zinc salts and/or hydrates thereof, magnesium salts and/ordifferent hydrates thereof and/or double salts.
 22. A process for theproduction of an absorption medium with a reduced caking tendency in amoist environment and/or at high temperatures, comprising the steps of:providing a component I which is a water- or aqueous fluid-absorbing,natural polymer modified with acid groups or a water- or aqueousfluid-absorbing, cross-linked polymer based on polymerized monomerscontaining at least partially neutralized acid groups; coating componentI with a coating agent selected from the group consisting ofnitrogen-containing, non-ionic surfactants as component II andoptionally a Lewis acid as component III; and heat treating the themixture.
 23. The process according to claim 22, further comprising thestep of mixing an aqueous solution of the coating agent(s) withcomponent I.
 24. The process according to claim 22, further comprisingthe step of treating the polymer before, during or after its surfacecross-linking with an aqueous solution of component II and optionalcomponent III, with the addition of heat.
 25. The process according toclaim 22, further comprising the step of simultaneously transforming theaqueous solution of component II and optional component III and surfacecross-linking the polymer.
 26. The process according to claim 22,further comprising the step of heating the mixture of components I andII to temperatures of 150° C. to 250° C., preferably 150° C. to 210° C.27. The process according to claim 22, further comprising the step ofheat treating the mixture of components I, II and III to temperatures of40° C. to 250° C., preferably 100° C. to 230° C., particularlypreferably 130° C. to 210° C.
 28. An absorption medium produced by theprocess defined in claim
 22. 29. Diapers, incontinence products foradults, feminine hygiene articles containing absorption mediums,preferably in composites, comprising: I a water- or aqueousfluid-absorbing natural polymer modified with acid groups or awater-insoluble, optionally surface cross-linked, water- or aqueousfluid-absorbing cross-linked polymer based on polymerized monomerscontaining at least partially neutralized acid groups, which is treatedwith: II at least one coating agent selected from the group consistingof nitrogen-containing, non-ionic surfactants; wherein the mixtureformed from components I and II has been heat treated; and wherein theabsorption medium doesn't show anti-caking after at least 3 hours ofheat treatment.